As a conventional image matching method using three-dimensional point group data, an ICP (iterative closest point) method is present. The ICP method is a technique for estimating, in two sets of three-dimensional point group data (G1, G2) to be image matching targets, by sequential and iterative calculation, using the sum of distance values between corresponding points as an error function, based on least-squares estimation, suitable position/attitude conversion parameters between the three-dimensional point group data (translation amount and rotation amount), so as to minimize the error function.
In this ICP method, because all combinations of the corresponding points are calculated, explicit corresponding information between the matching data is not needed. That is, information that a point in the three-dimensional point group data G1 and a point in the three-dimensional point group data G2 are identical is not needed. Therefore, images can be matched without teaching anatomical characteristic points, etc., by a human. Image matching methods using the ICP method are applied to various fields (for example, Patent Document 1); in radiation treatment, by using as reference three-dimensional CT image data for a treatment plan obtained when the radiation treatment is planned, an image according to this data and that to three-dimensional CT image data for a positioning operation, with respect to a bone structure, obtained during treatment are matched, and the position/attitude conversion parameters for matching both the images of the data is obtained, whereby patient positioning at the time of the treatment can be achieved.